This invention relates to surface peening of metal workpieces, and, more specifically, to laser shock peening thereof.
Fatigue strength of metal parts may be improved by introducing compressive residual stresses in the surface of such parts. This is typically accomplished by shot peening the surface with small metal shot to plastically deform and compress the metal surface layer. The metal surface must be uniformly peened to ensure the efficacy of the peening process.
Laser shock peening (LSP) is a modern development in peening, and uses a high peak-power laser to generate mechanical shock waves in order to produce compressive residual stresses in the metal surface. This process is performed by applying an absorbing material over the metal surface, such as black paint or tape, to absorb energy from the laser beam and produce a rapidly expanding or exploding plasma. The plasma produces a shock wave capable of plastically deforming the metal surface to introduce residual compressive stress in the surface.
LSP is substantially improved by locally confining the expanding plasma to concentrate the explosive pressure against the metal surface. This is typically accomplished by covering the metal surface with a thin confining layer of water which flows continuously over the surface and is thus replenished as LSP is conducted.
High power in the laser is produced by operating the laser in pulse-mode at a suitable repetition rate or pulse rate. In this way, the energy in each pulse may be maximized for maximizing the shock peening effect, while permitting replenishment of the confining water film between successive laser beam pulses.
Since each laser pulse produces a small explosion at the peening site, the water film is temporarily disrupted. Accordingly, the repetition rate of the power laser must be sufficiently low to ensure that a suitably thick and smooth water film is re-established after each peening pulse for efficiently confining successive pulse shots. If the repetition rate is excessive and the water film is not re-established promptly, the succeeding laser pulse will not be suitably confined, thus degrading the peening process and reducing its overall efficiency.
A high power pulse-mode laser configured for LSP is optimized for performance under a high average power thermal loading. Such pulse lasers operate at maximum efficiency at a corresponding pulse rate. However, operating the pulse laser below its designed pulse rate to ensure re-establishment of the confinement film results in poor laser performance.
There is presently no practical method of monitoring the quality of the water film to ensure its effectiveness for LSP. Care must be taken in the LSP process to ensure that a suitable film of water flows over the workpiece surface and is suitably re-established after each laser pulse. This is presently accomplished by visual observation of the water film and limiting the pulse rate to no greater than about one pulse ever four seconds to ensure re-establishment of a suitable confinement water film.
Accordingly, it is desired to provide a laser shock peening system including automatic monitoring of the confinement film to increase the repetition rate of the laser pulses and improve efficiency of the LSP process.